[Resolved] Registration page background messed up

Hello to everybody! I’ve got an issue with my background, on the registration page. For some reason seems like the page “doesn’t see” the registration fields, so the background has a really low height as you can see here: http://www.fbimprese.it/registra/

Any idea bout how to solve this?? I’ve already checked the CSS but I cannot set the height to a fixed value (for example to 1000px) because this configuration will be valid for all the pages in the website… Any fix? Any ideas?